What a fun night I've had! I've expanded my little test board to now have 4 LED's, but now I've also added a single byte of write only memory via the 72LS373 chip. This means that when I poke 1, the first led lights up, 2 the second, 3 1 and 2 and so on! So cool! You can't really see on the picture, but theres 2 LED's on and 2 LED's off. I did this simply by typing "POKE 65024,10" in BASIC. Now, I know this could be done via the userport, but 8 couldn't! And this is the basic stuff I need to be able to add lots of cool stuff!.
When I finally get my RAM, I hope to try and add external memory using basically the same idea, and having a RAM bank register just like this! I'm NOT looking forward to wiring the thing up though!
I had a few problems adding this to the old design though. First was again a simple case of not fully checking the IC pinouts, it really does get tricky when the IC is upside down and back to front! But there was also the problem of the 6502 using a 2 phase clock, which meant I had to check that line as well as the R/W line before latching the data, and that the Write line was basically inverted from what I though it should have been - bit of a pain that. So I had to add another chip, this time a proper AND chip (I got some yesterday from a Maplin store). I also had to figure out how to use the 373 chip as a normal latch to store my data, which meant building a new mini breadboard circuit to try things out on. However, once I figured it all out, I was able to turn my LED's on and off via a memory location in the I/O space. Oh! And whoever wrote the little IO text file describing whats where - THANKS! Helped loads!!
So...whats next! I guess add the other 4 data lines so I have a full byte there, and then perhaps hook up the LCD screen to it and play with having the Plus/4 drive it! Although, after all of tonights work, that should be easy - the data's there, its just a matter of soldering it all into place. I wonder how hard a SID chip add on would be to build, or a memory mapped MMC card. Once you've done this, the rest should be easy - although, I've not done memory reading yet, this is a write only register.
I'd also just like to point out to anyone that would like to try this kind of thing, that I've only just started doing electronics - just a couple of months really, and once you get past all the resistors and other do-hickys, its all pretty simple. Hooking up chips is really easy, particually out the back of a retro computer! Anyway, if you really fancy learning, I'd highly recomend Electronics for Dummies (ISBN 0-7645-7660-7), which is the main book that got me started, and The BLACK ART of Video Game Console Design(ISBN 0-672-32820-8), which has lots of info on how basic computers work. This book is part of the XGameStation setup, which is also fab. Its a little heavy going at the start delving into areas you just don't need to know about, but the information past that is invaluable!
Although...a small word of warning - it ain't cheap to start! Parts are pretty cheap, $0.30 for a cheap here, $0.95 for a PIC there, but you really need a small stash of things, and these add up really quickly! And don't get me started about if you want to make your own boards!!